Foundation structures are commonly used to locate objects on sea beds and river beds, and are particularly suited to the installation of tidal turbines. One common type of foundation structure is a gravity base structure. A gravity base structure is normally towed to the location at which installation of an object on a sea bed or river bed is desired and includes ballast tanks which can be filled or emptied with suitable ballast material to control the buoyancy of the gravity base structure. The gravity base structure can be lowered in the water, towards the sea bed or river bed, for example by flooding the ballast tanks with water and can be raised from the sea bed or river bed by evacuating water from the ballast tanks if it becomes necessary to retrieve the object mounted on the gravity base structure from the sea bed or river bed.
It is important to be able to control the stability of a foundation structure such as a gravity base structure during the submergence (deployment) and surfacing (retrieval) operations and this requires careful control of the position of the centre of buoyancy of the structure relative to the position of the centre of gravity of the structure. Gravity base structures with permanent structural features that contribute to such stability have been proposed. However, these permanent structural features can affect the stability and/or performance of the gravity base structure when it is located in its final position on a sea bed or river bed, in particular due to increased drag. For example, permanent structural features in the form of vertical extensions have been used to control the stability of a gravity base structure during submergence of the structure, but those permanent vertical extensions render the gravity base structure unsuitable for use in locations where there are strong tidal currents.
There is a need for a foundation structure which has improved stability during submergence and surfacing and which has acceptable stability and performance when located on a sea bed or river bed.